The invention is directed to a method for producing containers made of a thermoplastic material that cannot be permanently electrostatically charged and from which electric charges can be drained, and to a plastic container produced according to the method with a permanently electrostatically non-chargeable and/or electric charge-draining coating, for storing and transporting liquid fill material, in particular for flammable or potentially explosive fill materials.
A plastic container of this type typically has vertically oriented container walls with a horizontal container top wall and container bottom wall, wherein a closable fill and/or drain port that is a gas-tight and liquid-tight is arranged at least on the container top wall or on the container cover. The plastic container can be configured, for example, as a canister, barrel, a tight-head drum, a drum with removable cover and clamping ring, or as an inner container of a pallet container.
For use or application with flammable or potentially explosive fill materials, plastic containers are frequently—at least in the exterior layer—colored black with electrically conducting carbon black for obtaining a surface that cannot be permanently electrostatically charged and from which electrostatic charges can be drained.
In general, it is desirable that plastic containers with a colored exterior layer be able to indicate the respective fill or drain state of the container. Such plastic containers typically have a narrow vertical strip for visual inspection, indicating the respective fill level of the fill material in the container. Manufacture of plastic containers with a visual inspection strip by a blow molding process is fairly complex and requires an extrusion head equipped with an additional extruder for the material of the visual inspection strip.
A conventional pallet container (=fluid container of this type) is disclosed, for example, in DE 196 05 890 A1 (Pro. Feb. 19, 1996). When a fluid container of this type is filled or drained, or when the liquid fill material sloshes back and force due to movement in transit, or when fluids in these plastic containers are stirred, for example, for the purpose of mixing, electric charges generated by friction between the fluid and the container surface are discharged to ground by—as referred to in this publication—a permanent antistatic or an electric charge-draining exterior layer of the inner container and the metallic support frame (steel pallet).
The permanent antistatic coating is here made of a plastic having an additive of conductive carbon black (this is generally known as an electric charge-draining, but not as an antistatic coating!). This prevents an electric discharge, possibly accompanied by spark generation, between the plastic inner container and the metallic lattice cage, which can cause ignition of potentially flammable fill materials of the pallet container or of potentially explosive mixtures of gases and vapors.
The known pallet container with a colored or blackened exterior layer (conductive carbon black) disadvantageously precludes a visual inspection of the fill level without an inspection strip due to the outer coloration of the plastic inner container. The addition of carbon black also significantly reduces the mechanical strength of the plastic material, in particular by reducing the resistance to fracture and its weldability.
Another pallet container disclosed in DE 38 44 605 (Sch. Jun. 11, 1988) has at least one inspection strip made of a translucent or transparent plastic material which forms a unit with the black colored plastic material (single layer material continuously colored).
Moreover, DE 41 36 766 (Ro. Nov. 8, 1991) discloses a multi-layer container wall with a colored exterior container layer having a discontinuous region in the layer in the form of a strip. The strip-shaped discontinuous region of the layer is materially connected with the colored plastic material of the outer container layer.
Another pallet container with a multi-layer inner container, with a permanent antistatic exterior layer colored with carbon black and with at least one integrated visual inspection strip made of a transparent plastic material is known from DE 202 06 436 (Pro. Apr. 23, 2002).
All of the aforedescribed colored plastic containers have a continuous visual inspection strip which is arranged in the container wall and made of the colorless plastic material of the interior layer. The manufacture of these plastic containers involves a complex process and typically requires an additional extruder. If the inspection strip is combined with an adjoining volume scale, the circumferential location of the inspection strip on the inner container must exactly match the circumferential location of the volume scale. In addition, exact vertical alignment of the inspection strip on the inner container is difficult to maintain during the manufacture of the inner container, i.e., during blowing and stretching of the tubular preform in a cuboid blow mold.